The present invention relates generally to crosstalk cancellation techniques and more particularly, to methods and apparatus for reducing near-end crosstalk (NEXT) and echo crosstalk in the continuous time domain.
FIG. 1 illustrates a hybrid transceiver 100 that transmits and receives signals on the same twisted pair (TP) 110. The hybrid transceiver 100 is commonly associated with a local area network (LAN) or digital subscriber loops (xDSL). The main sources of crosstalk in such a transceiver 100 are usually near-end crosstalk (NEXT) and Echo crosstalk. Each hybrid transceiver, such as the transceiver 100, transmits a first signal, V1, and receives a different signal, V2, on the same twisted pair 110. Near-end crosstalk (NEXT) results from transmitting and receiving different signals on different twisted pairs 110 and having a signal on one twisted pair interfering with the signal on another twisted pair. V1 corresponds to the transmitted signal generated by the transceiver 100. V2 corresponds to the received signal generated by a second transceiver 120. Since the transceiver 100 knows the transmitted signal, V1, that it has generated, the transceiver 100 can subtract the transmitted signal, V1, from the voltage (V1+V2) on the twisted pair (TP) 110, to obtain the voltage corresponding to the received signal V2.
Echo crosstalk, on the other hand, is the result of crosstalk on the same twisted pair 110 and of discontinuous impedances along a given path, for example, at each connector. When the transceiver 100 transmits a signal, V1, each impedance discontinuity along the path causes the transceiver 100 to receive a wave or echo back. FIG. 2 corresponds to the impulse response 200 of an impulse signal transmitted along the twisted pair (TP) 110 by the transceiver 100. Generally, each peak 210-213 in the impulse response 200 corresponds to a different connector (not shown) on the twisted pair (TP) 110. The impulse response 200 is utilized to obtain the echo path and then adjust the taps of an echo canceller (not shown) to the peaks of the echo impulse response. The taps are adjusted, for example, using the well known least mean square (LMS) algorithm, to match the energy and delay of the echo canceller to the peaks of the echo impulse response.
If the echo impulse response is not dynamic, the tap values converge and remain constant. In a synchronous communication system, for example, the impulse response is static and coincides w with the symbol rate. Thus, the echo canceller can be implemented using low cost, low power digital circuitry. In many applications, however, the transmitter and receiver are not synchronized and the resulting impulse response is dynamic. In such an asynchronous communication system, a frequency offset exists between the transmitter and the receiver.
As apparent from the above-described deficiencies with conventional crosstalk cancellation techniques, a need exists for a finite impulse response (FIR) filter that emulates the crosstalk and tracks the frequency offset of asynchronous communication systems. A further need exists for a clock recovery circuit to recover the clock of the received signal, to determine when to sample the received signal.
Generally, a mixed-mode crosstalk canceller is disclosed that performs crosstalk cancellation in the continuous time domain. The disclosed mixed-mode crosstalk canceller processes the pulse amplitude modulated (PAM) digital signal to be transmitted as well as the received signal to compensate for the crosstalk between the transmit and receive signals. In accordance with one aspect of the invention, the output of the crosstalk canceller is subtracted from the received signal in the continuous time domain. Thus, the transmit symbol clock and the receive symbol clock can be asynchronous.
In the illustrative embodiment, the tap weights for the crosstalk cancellation are obtained using a modified version of the least mean square (LMS) algorithm for discrete time signals. The modified least mean square (LMS) algorithm is applied for continuous time signals that are derived from different clocks, using the following equation:
wk(n+1)=wk(n)+xcexc{tilde over (e)}(t)xc2x7xk(t)
Thus, the LMS algorithm requires a costly multiplication of the error signal, e(t), and the digital transmit signal. xk(t). Since it is only necessary to go in the direction of the gradient with the steepest decent, however, computational gains are achieved in accordance with the present invention using a correlation multiplier that quantizes e(t) and xk(t) with only one or two bits, and performs the correlation multiplication using an asynchronous logic circuit. In the illustrative embodiment, the correlation multiplier receives a 2 bit quantized representation of the digital transmit signal. xk(t), and a 1 bit quantized version of the error signal, e(t). The quantized version of the error signal, e(t), indicates the sign of the error (positive or negative) and is obtained in the illustrative embodiment by comparing the error signal to zero.
A crosstalk canceller is disclosed that updates each tap weight utilizing the disclosed correlation multiplier that provides a signal indicating whether the tap weight needs to be increased or decreased to a charge pump that produces a current in-the proper direction.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.